Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the recording element, while un-imaged droplets are caught and returned to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
It is well known in the art that printed images generated using ink jet technology are not very stable with regard to environmental effects of light and ozone. Another problem with such printed images is that they are easily scratched during handling and are not resistant to typical aqueous solutions such as coffee, fruit punch, etc.
Several methods have been used to protect printed images generated using ink jet technology. One method is referred to as lamination and involves the application of a clear durable film over the printed image. Another method involves printing onto specially designed ink jet recording elements which are subsequently fused to form a clear durable film over the printing image. Both of these methods have drawbacks in that an integral or peripheral fusing station is required.
Another method for increasing the durability of printed images involves application of an overcoat composition onto the surface of the printed image using a brush, roller, sponge, etc. As the composition dries, a clear durable film is formed. This method is useful in a variety of commercial printing applications but is considered too impractical and undesirable for consumer use in the home. To this end, the use of a colorless ink supplied in a printhead of an ink jet printer has become increasingly popular. The printhead containing the colorless ink is typically part of the same carriage assembly containing colored inks, and the printer is instructed to jet the colorless ink either simultaneously with or after the colored inks are jetted.
U.S. Pat. No. 6,087,051; U.S. 2003/0193553 A1; U.S. 2003/0005945 A1; JP 2003-291484 A; JP 2000-225695 A; and JP 2002-144551 A describe methods of ink jet printing wherein a colorless ink is used to equalize gloss and/or provide image permanence. U.S. 2002/0156153 A1 describes the use of polymeric additives in fixatives for ink jet printing in order to improve print quality and image permanence attributes on plain paper. This application discloses that a wide variety of polymers can be used, as long the polymer glass transition temperature (Tg), melting temperature, and molecular weight each fall into a particular range. The fixatives, however, require high amounts of organic solvents for jetting, making the fixatives environmentally unfriendly and unsuitable for use on glossy ink jet recording elements.
U.S. Pat. No. 6,604,819 B2 relates to an ink jet image recording method that includes the application of a solution having fine polymer particles during or after printing colored inks in order to provide weather fastness. This patent states that the particles must have a weight average molecular weight of at least 100,000 such that a film is formed therefrom. However, polyurethanes having a weight average molecular weight of at least 100,000 are difficult to jet, especially from a thermal printhead.
U.S. 2002-0009547 A1 relates to a coating liquid for application to recorded images in order to provide image permanence, fixation and glossiness. This application discloses that fine polymer particles may be used in the coating liquid, and that it is especially desirable if the particles have an acid number of 100 or less. This application does not describe how the physical properties of polymers must be optimized in order to obtain jettability, stain resistance and rub resistance.
Several prior arts, such as US 2004/0030001, US 2004/0017406, US2004/0032473, US2003/85974 and US2003/189626 teach the use of a substantially colorless ink in the non-imaged areas of a print to reduce the gloss difference between the image and non-imaged area. The materials used in the colorless ink vary from inorganic particles to organic particles to water soluble polymers, but none of them teaches an ink formulation that can offer stain protection for a variety of ink jet media.
Therefore, there is a still a need for a coating composition formulation that can be jetted from an ink jet printhead reliably and can offer stain protection for commonly used inkjet media in the trade.